What is the primary advantage of using membrane bioreactors (MBRs) over conventional activated sludge systems?

The primary advantage of using membrane bioreactors (MBRs) over conventional activated sludge systems lies in their significantly improved effluent quality, specifically the near-complete removal of suspended solids and consistently lower concentrations of dissolved organic matter. To understand this, it's crucial to define the key components of both systems. A conventional activated sludge system treats wastewater through a biological process where microorganisms, primarily bacteria, consume organic pollutants in a tank called an aeration basin. Air is pumped into the basin to provide oxygen for these bacteria, which form what's known as 'activated sludge.' This sludge, along with the treated wastewater (effluent), flows to a sedimentation tank, or clarifier, where the solids settle out. This settled sludge is then partially returned to the aeration basin to maintain a healthy microbial population, and the excess sludge is removed as waste. The effluent leaving the clarifier, however, still contains some suspended solids and dissolved organic compounds, requiring further treatment if stringent discharge standards are needed.

An MBR combines the biological treatment of an activated sludge system with a membrane filtration process. The 'bioreactor' part is essentially the same aeration basin with microorganisms consuming pollutants. However, instead of a sedimentation tank, MBRs use membranes to separate the treated water from the biomass. These membranes are very fine filters, typically made of polymeric materials, with pore sizes ranging from 0.01 to 0.1 microns. This means they can retain virtually all suspended solids, including bacteria, viruses, and even some very small particles that would pass through a conventional clarifier. There are two main types of membrane configurations: submerged, where the membranes are directly immersed in the aeration basin, and side-stream, where the mixed liquor (wastewater and biomass) is pumped to a separate membrane filtration unit.

The key benefit stems from this membrane separation. Because the membranes retain virtually all solids, MBRs produce an effluent with extremely low suspended solids concentrations – often below 1 mg/L, compared to 10-30 mg/L typically achieved in conventional systems. Furthermore, the membranes also act as a barrier to many dissolved organic compounds, leading to a reduction in Chemical Oxygen Demand (COD) and Biological Oxygen Demand (BOD) in the effluent. COD and BOD measure the amount of organic matter in the water that can be broken down by microorganisms, and lower values indicate cleaner water. For example, a conventional system might achieve 80-90% COD removal, while an MBR can achieve 90-95% or higher. This superior effluent quality allows MBRs to meet stricter discharge regulations, enable water reuse applications (like irrigation or industrial cooling), and potentially reduce the need for downstream treatment processes like disinfection. The higher biomass concentrations possible in MBRs due to the membrane retention also lead to a smaller footprint for the treatment plant, as less space is needed for sedimentation.